"Now the torch as been passed," Obama said, turning to Harris' candidacy and her campaign against former President Donald Trump.

"Now it is up to all of us to fight for the America we believe in," he added. "And make no mistake, it will be a fight."

Obama went on to slam Trump as a "78-year-old billionaire who has not stopped whining about his problems since he rose down his golden escalator."

"As we gather here tonight, the people who will decide this election are asking a very simple question: Who will fight for me? Who’s thinking about my future, about my children’s future, about our future together?" he asked. "One thing is for certain, Donald Trump is not losing sleep over that question."

As Obama reminisced on accepting the party's nomination 16 years ago, he spoke about choosing President Joe Biden to be his running mate.

"And looking back, I can say without question that my first big decision as your nominee turned out to be one of my best, and that was asking Joe Biden to serve by my side as vice president," he said.

"Other than some common Irish blood, Joe and I come from different backgrounds," he continued. "When we became brothers, and as we worked together for eight, sometimes pretty tough, years. What I came to admire most about Joe wasn’t just his smarts; his experience. It was his empathy, and his decency. And it’s hard earned resilience. His unshakable belief that everyone in this country deserves a fair shot. And over the last four years, those are the values America has needed most."

Obama went on to praise Biden's leadership as president and for putting the nation before his personal ambition by stepping aside.

"History will remember Joe Biden as an outstanding president who defended democracy at a moment of great danger," Obama said. "And I am proud to call him my president, but I am even prouder to call him my friend."

2:58

Barack Obama praises Biden as 'a leader who was steady'

The former president said of Biden: "I am proud to call him my president, but I am even prouder to call him my friend."

The former president received a warm welcome from the crowd in the United Center, who revived his iconic campaign slogan "Yes we can!"

"I'm feeling fired up!" Obama said. "I am feeling ready to go -- even If I am the only person stupid enough to speak after Michelle Obama."

Michelle Obama reminds everyone why she’s on of the Democratic Party’s most popular figures. She consistently polls as someone Democrats would like to see run for president -- and this speech showed why.

-ABC News' Tal Axelrod